A silicon-on-insulator type substrate comprises a semiconductor film, for example made of silicon or a silicon alloy, for example a silicon-germanium alloy, located on a buried insulating layer, commonly designated by the acronym “BOX” (for Buried OXide), itself located on a carrier substrate, for example a semiconductor well.
In a fully-depleted SOI (FDSOI) technology, the semiconductor film is fully depleted, i.e. it is composed from an intrinsic semiconductor. Its thickness is generally about a few nanometers, for example 7 nanometers. Moreover, the buried insulating layer itself generally has a small thickness of about twenty nanometers.
On account of the small thickness of the semiconductor film, the source and drain regions of transistors comprise portions that are raised relative to the semiconductor film so as to ensure an adequate electrical connection between these regions and the channel region of the transistor.
Such raised source and drain regions (commonly designated in the art by the acronym “RSD”: for Raised Source and Drain) are typically obtained by epitaxy.
Moreover, the production of transistors capable typically of withstanding high voltages of about a plurality of volts, such as for example extended drain MOS transistors (known in the art by the acronym “DRift MOS”), requires thick gate oxides to be formed.
However, this proves to be complicated to achieve on SOI and in particular FDSOI type substrates because of the small thickness of the semiconductor film.
Specifically, the semiconductor film will be partially consumed during the production of these thick oxides. Furthermore, since it is necessary to preserve the initial thickness of the semiconductor film (for example 7 nanometers), it is then necessary to start with a thicker semiconductor film the thickness of which must be adjusted to obtain in fine said initial thickness on account of the expected consumption of the semiconductor film.